The present invention relates to a system for detecting a rotation angle of a deflection mirror.
Conventionally, an optical data recording/reproducing device for recording/reproducing optical data on/from an optical disc using an optical head. When the optical data is read, a laser beam emitted by a laser diode is converged on the optical disc by an optical system, and a reflected beam is received by a light receiving element. When the data is read, the optical head is moved in a radial direction to follow a track formed on the optical disc.
Since the disc spins at a relatively high speed, and the center of a circle defined by the tracks may be displaced from the center of spin, it is relatively difficult to move the entire optical head to accurately follow the tracks.
In order to accurately adjust the position of the optical head with respect to a track, in a conventional device, the beam incident on an objective lens of the optical head is made inclined. For this purpose, an optical system employing a deflection mirror has been suggested. In such a system, the deflection mirror is provided within an optical path between the laser diode and the objective lens. When the deflection mirror rotates about its rotation axis, the direction in which the beam is incident on the objective lens is changed. Accordingly, by controlling the rotation angle of the deflection mirror, the position of the beam spot on the optical disc can be adjusted. That is, by moving the optical head to roughly locate the beam spot in the vicinity of a target track (i.e., a primary tracking is performed), and then by driving the deflection mirror, tracking position is adjusted accurately (i.e., a fine tracking is performed). In such a system, when the primary tracking is performed, the deflection mirror is positioned at its reference or neutral position, and when the fine tracking is performed, the rotation angle of the deflection mirror with respect to the reference position is controlled. Generally, by rotating the deflection mirror, the beam spot moves across from several tracks to several tens of tracks (i.e., several micrometers).
In such a system, however, if the deflection mirror is rotated to a position out of a predetermined rage, optical performance of the disc drive may be deteriorated. Thus, the rotation angle of the deflection mirror should be monitored and controlled so that the rotation angle of the deflection mirror does not exceeds a predetermined angular range.
For detecting the rotation angle of the deflection mirror, a rotation angle detecting system is provided. An example of such a system is provided with a light emitting element and a pair of light receiving elements. In such a system, light is projected onto a surface which is different from a reflection surface of the deflection mirror, and light reflected thereby is received by a pair of light receiving elements. By comparing the output signals of the pair of light receiving elements, the rotation angle can be detected. If such a detection system is used, however, two light sources are necessary. That is, a light source used for reading/writing data, and another light source used for detecting the rotation angle of the deflection mirror should be provided. Therefore, if such a system is employed in an optical disc drive, the size thereof becomes relatively large, and a manufacturing cost may also increase.
It is therefore an object of the invention to provide an improved rotation angle detection system for detecting a rotation angle of a deflection mirror provided in an optical head of an optical disc drive.
For the object, according to the invention, there is provided a rotation angle detecting system for detecting a rotation angle of a deflection mirror employed in an optical disc drive. The rotation angle detecting system is provided with: a light source that emits a light beam to be incident on an optical disc; a reflection member provided on the deflection mirror; a beam splitter that splits the beam emitted by the light source into a first beam directed to the optical disc via the deflection mirror and a second beam directed to the reflection member; and a detector that receives the second beam reflected by the reflection member and determines the rotation angle of the deflection mirror in accordance with the second beam.
Since the light beam which is to be converged on the optical disc is split and used for detecting the rotation angle of the deflection mirror, another light source to be used only for detecting the rotation angle of the deflection mirror is not necessary.
Preferably, the reflection member comprises a portion of a reflection surface of the deflection mirror. In this case, an another reflection member only for detecting the rotation angle of the deflection mirror is not necessary.
Optionally, the detector may be a single element including two light receiving areas, each of the two light receiving areas outputting signal representing amount of received light, the two light receiving areas being arranged in a direction where the second beam incident on the two light receiving areas moves when the deflection mirror rotates. The detector determines the rotation angle of the deflection mirror based on the output signals of the two light receiving areas.
Particularly, the detector may be a photodiode having two light receiving areas.
Optionally, the beam splitter may be a beam splitting prism, a half-mirror surface being formed inside the beam splitting prism. In this case, the first beam may pass through the half-mirror surface, and the second beam may be reflected thereby.
Further optionally, the light source may be provided with a laser diode emitting a diverging laser beam having an elliptical cross section; and a collimator lens that converts the diverging laser beam into a parallel laser beam. In this case, a surface of the beam splitting prism on which the light beam emitted by the laser diode is incident is inclined with respect to an optical path of the incident laser beam so that the first beam has a circular cross section. In other words, the beam splitting prism also functions as a beam shaping prism.
In particular case, the beam splitting prism may be constituted such that the second beam is reflected a plurality of times inside the beam splitting prism.
Still optionally, the detector detects the rotation angle of the deflection mirror in accordance with a difference between the output signals of the two light receiving areas.
Yet optionally, the detector may detect an intensity of the light beam emitted by the light source in accordance with a sum of the output signals of the two light receiving areas.
In this case, the optical disc drive may be provided with a controller that adjusts the intensity of the light beam emitted by the light source in accordance with the intensity of the light beam detected by the detector. Thus, with use of a single detector, both the rotation angle of the deflection mirror and the intensity of the light beam can be detected.
According to another aspect of the invention, there is provided a head of an optical disc drive, which is provided with: a light source that emits a light beam; an objective optical system that receives the light beam emitted by the light source, the light beam passed through the objective optical system being converged onto an optical disc; a deflection mirror provided between the light source and the objective optical system, the deflection mirror being rotatable about a predetermined rotation axis, the light beam emitted by the light source being incident on a reflection surface of the deflection mirror and reflected towards the objective optical system; a reflection member provided on the deflection mirror; a beam splitter that splits the light beam emitted by the light source into a first beam that is directed to the optical disc via the deflection mirror and a second beam that is directed to the reflection member; and a detector that receives the second beam split by the beam splitter and reflected by the reflection member, the detector determining the rotation angle of the deflection mirror in accordance with the received second beam.